The combustion of coal and other fossil fuels in a power generating unit causes buildup of combustion deposits (e.g., soot, ash and slag) in the boiler, including boiler heat transfer surfaces. Combustion deposits generally decrease the efficiency of the boiler, particularly by reducing heat transfer. When combustion deposits accumulate on the boiler tubes, the heat transfer efficiency of the tubes decreases, which in turn decreases boiler efficiency. To maintain a high level of boiler efficiency, the heat transfer surfaces of the boiler are periodically cleaned by directing a cleaning medium (e.g., air, steam, water or mixtures thereof) against the surfaces upon which the combustion deposits have accumulated.
To avoid or eliminate the negative effects of combustion deposits on boiler efficiency, the boiler heat transfer surfaces would need to be essentially free of combustion deposits at all times. Maintaining this level of cleanliness would require virtually continuous cleaning. However, this is not practical under actual operating conditions because cleaning is costly and creates wear and tear on boiler surfaces. Injection of the cleaning medium can reduce boiler efficiency and prematurely damage heat transfer surfaces, particularly if they are over cleaned. Boiler surface and water wall damage resulting from cleaning is particularly costly because correction may require an unscheduled outage of the power generating unit. Therefore, it is important that these surfaces not be cleaned unnecessarily or excessively.
Boiler cleanliness must be balanced against cleaning costs. Accordingly, power generating plants typically maintain reasonable, but less than ideal boiler cleanliness levels. Cleaning operations are regulated to maintain the selected cleanliness levels in the boiler. Different areas of the boiler may accumulate combustion deposits at various rates, and require separate levels of cleanliness and different amounts of cleaning.
The devices used for cleaning the boiler heat transfer surfaces are commonly referred to as soot cleaning devices. Fossil fuel-fired power generating units employ soot cleaning devices including, but not limited to, sootblowers, sonic devices, water lances, and water cannons or hydro-jets. These soot cleaning devices use steam, water or air to dislodge combustion deposits and clean surfaces within a boiler. The number of soot cleaning devices on a given power generating unit can range from several to over a hundred. Manual, sequential and time-based sequencing of soot cleaning devices have been the traditional methods employed to improve boiler cleanliness. These soot cleaning devices are generally automated and are initiated by a master control device. In most cases, the soot cleaning devices are activated based on predetermined criteria, established protocols, sequential methods, time-based approaches, operator judgment, or combinations thereof. These methods result in indiscriminate cleaning of the entire boiler or sections thereof, regardless of whether sections are already clean.
In recent years, some power generation plants have replaced manual or time-based systems with criteria-based methods, such as cleaning the boiler in accordance with maintaining certain cleanliness levels. For example, one common approach is to attempt to maintain a predefined cleanliness level by controlling the soot cleaning devices. After a soot cleaning device has cleaned a surface, one or more sensors measure the resulting heat transfer improvement and determine the effectiveness of the immediately preceding soot cleaning operation. The measured cleanliness data is compared against a predefined cleanliness model that is stored in a system processor. One or more soot cleaning operating parameters can be adjusted to alter the aggressiveness of the next soot cleaning operation. The goal is to maintain the required level of heat transfer surface cleanliness for the current boiler operating conditions while minimizing the detrimental effects of the soot cleaning operation.
Criteria-based methods for soot cleaning have some drawbacks. To implement a criteria-based method, it is often necessary to install additional hardware in the boiler, such as heat flux sensors. In addition, cleanliness models are needed to adjust the performance of the soot cleaning control system. Developing these models can be challenging since the models are typically based upon rigorous first principle equations. Finally, criteria-based methods focus on cleaning specific zones in the boiler, rather than improving overall boiler performance.
Boiler operation is generally governed by one or more boiler performance goals. Boiler performance is usually characterized in terms of heat rate, capacity, emissions (e.g., NOx and CO), and other parameters. One principle underlying a soot cleaning operation is to maintain the boiler performance goals. The above-described criteria-based methods do not relate boiler performance to a required level of heat transfer surface cleanliness and, therefore, to optimum operating parameters. The approach assumes that the optimal cleanliness of an area in the boiler is known (e.g., entered by an operator). Accordingly, the approach assumes that required cleanliness levels for desired boiler performance goals are determined separately and provides no mechanism for selecting cleanliness levels for individual heating zones of the boiler. A criteria-based soot cleaning control system does not relate operational settings to boiler performance targets.
The present invention provides a soot cleaning control system that overcomes the drawbacks discussed above, as well as other drawbacks of prior art soot cleaning control systems.